Specific Aims: Study 1 1. To compare the concentration of articular cartilage markers (aggrecan, stromelysin, tissue inhibitor of metalloproteinases (TIMP), procollagen II C-peptide, cartilage and oligiometric matrix protein (COMP) in joint fluid aggrecan, COMP, and keratan sulfate in serum) obtained from joint fluid of both knees with a posterior horn meniscus tear with the contralateral uninjured knee at arthroscopy. 2. To study the relation between articular cartilage marker concentrations and the cartilage condition observed with arthroscopy and MRI. 3. To determine the relationship between the patients persisting symptoms and function and the concentration of cartilage markers in joint fluid obtained from knees with a symptomatic meniscus tear, before and after 6 mon. rehab. 4. To determine the relationship between the patients persisting sympoms and function and the concentration of cartilage markers in joint fluid obtained from the contralateral normal knee at hte same time intervals as used for the injured knee. Study 2: 1. To compare the concentrations of articular cartilage markers in joint fluid obtained from the right knee with concentrations of these markers in joint fluid from the left knee in subjects with normal MR images of their knees. 2. To compare the concentrations of articular cartilage markers in joint fluid obtained from the right and left knees in subjects with normal MR images of their knees with the concentrations of these markers in joint fluid from the injured knee and contralateral normal knee in patients with a meniscus tear. Joint fluid samples are brought to the GCRC Laboratory and are clarified by centrifugation. Aliquots are prepared and frozen in a GCRC freezer for later analysis.